Isn't the cruise capacity still limited by the number of rooms? So saving peanuts on drinks and cheap food probably doesn't make that big of a difference for them.

Well, they mostly don't. They're often using the same bedding. Most toiletries are squirt containers these days. I don't think toilet paper is that expensive. And another towel or two to wash is probably not a big deal. And to the degree the hotel has a restaurant or bar they probably come out ahead. Your hotel may take a different approach but it's near universal (perhaps outside of resorts) that hotels charge the same for 1 or 2 guests.

2 vs 1 doesn’t significantly impact space or cleaning labor, unless you’re staying in a super minimalist itty bitty unit (which I rarely find exists anymore)

If there's two beds, maybe. If one bed, doesn't matter how many people sleep in it. Maybe twice the towels?

I'll say over the years most places do not respect the "if the towel is not on the floor don't replace it". I'm fine with reusing a towel to dry off twice, but some hotels change them every day, even when their signage is indicating a protocol to prevent that sort of waste.